Arbitrary fees and price adjustments

Hey team, really loving FluentCart so far! This is a feature suggestion for a couple of things that WooCommerce does that I think would be useful in FluentCart, specifically for service businesses that regularly provide the same services at different prices for different clients.

Arbitrary price changes per line item
In a WooCommerce order (in blue in attached screenshot), you can arbitrarily increase or decrease the price of each line item. Currently in FluentCart, you can only apply a whole-order discount, and can't increase prices.

Arbitrary fees
In a WooCommerce order (in red in attached screenshot), you can add any arbitrary fee, which you can't currently do in FluentCart.

Example usage
I think this would be very useful for a web development agency, for example. They might have a standard website development cost, but need to increase or decrease the price on each invoice based on the size of the project.

I think if these features were available, alongside the recent hidden products discussion (https://community.wpmanageninja.com/portal/space/fluent-cart/post/unlock-hidden-products-secret-offers), FluentCart would be even better for service-based businesses!

Thanks again for all of the work on FluentCart 🌟